1. Field of the Invention
This invention relates to electrostatic powder coating systems, and more particularly, to the triboelectric charging of powder material for use in powder spraying systems.
2. Description of the Prior Art
In electrostatic powder spraying, particulate powder material is fluidized in a powder hopper and pumped through a hose to a spray gun which electrically charges the powder and sprays the powder onto a product to be coated. The powder particles sprayed from the gun are electrostatically attracted to the product being coated which is generally electrically grounded. Once these electrostatically charged powder particles are deposited onto the product, they adhere there by electrostatic attraction until the product is conveyed into an oven where the powder particles are melted to flow together to form a continuous coating on the product.
The spray gun typically charges the powder in one of two ways-either the gun has a high voltage charging electrode which produces a corona to charge the powder, or the gun has means to charge the powder by friction, i.e., triboelectrically. This invention relates to triboelectric powder spray guns.
Generally, in triboelectric powder guns contact surfaces are provided within the gun, typically constructed from an electrically insulating material, such as polytetrafluoroethylene (PTFE) or nylon, and the powder particles impact these surfaces numerous times to frictionally charge the particles. The powder particles are then sprayed from the front of the gun, where they are electrostatically attracted to the product to be coated which has been electrically grounded.
Triboelectric powder spray guns may be used to produce either a positive or negative charge on the powder depending upon the material being sprayed. The tribocharging preferences of certain materials are well known. For example, polytetrafluoroethylene (PTFE) and nylon have well known tribocharging tendencies which represent tribocharging extremes, with PTFE exhibiting strong negative tribocharging properties and nylon exhibiting strong positive tribocharging properties. Materials that exhibit a strong positive charging tendency and a weak negative charging tendency, such as nylon and some epoxy materials, are used as powder materials in a positive triboelectric spraying system. Materials that exhibit a strong negative charging tendency and a weak positive charging tendency, such as PTFE and some polyester materials, are used as powder in a negative tribocharging spraying system.
Materials which would be used as powders in a positive triboelectric spraying system can be used as contact surfaces in a negative triboelectric spraying system and vice versa. Thus materials that exhibit a strong negative charging tendency, such as PTFE, are used as contact surfaces in a positive triboelectric spraying system, and materials that exhibit a strong positive charging tendency, such as nylon, are used as contact surfaces in a negative tribocharging spraying system.
Various designs for triboelectric spray guns are known, such as those shown in U.S. Pat. No. 4,399,945 and application Ser. No. 07/956,615. These guns and other similar guns are also commercially available, such as the guns available as Tribomatic.RTM. guns from Nordson Corporation, Amherst, Ohio. In these guns, the powder can be triboelecrically charged in various ways, such as in a bundle of curved PTFE tubes which are wrapped around a core, or through a wavy undulating path formed in an annular gap between an outer cylinder and a central core each having PTFE contact surfaces. As the powder passes through the flow path, it impacts the contact surfaces several times and picks up charge upon each contact. The contact surfaces are provided with a ground path to bleed the charge on the surfaces to ground and avoid a buildup of charge on the contact surfaces during operation of the gun. In addition, grounding of the gun is needed for reasons of safety to prevent the gun from storing a capacitive charge which could shock an operator or produce a spark, causing a fire or explosion.
In normal tribocharging with a positive tribocharging powder, such as many epoxy materials, the powder impacts the contact surface made of a material such as PTFE. As the powder becomes tribocharged, it gives up electrons to the PTFE, and the powder becomes positively charged. When the concentration of negative charges builds up on the PTFE contact surface to a certain level, the negative charges find the nearest ground to discharge the electron buildup. If the electrons cannot be discharged, the tribocharging process is disrupted, and no additional powder can be charged.
In order to facilitate the discharge of electrons from the contact surface, it has always been important to provide an effective grounding path in triboelectric powder spray guns. Various solutions to providing an effective grounding path have been proposed. However, all of these solutions must prevent inadvertent grounding of the powder, either through buildup of powder around the ground or through direct grounding of the contact surfaces themselves. Ground paths must, therefore, be somewhat complicated, and grounding of prior art guns sometimes involves time-consuming and complicated manufacturing processes.